Method for manufacturing a steel sheet

ABSTRACT

The method for manufacturing steel sheet comprises the steps of: rough-rolling to form a sheet bar; finish-rolling the sheet bar to form a steel strip; applying primary cooling and secondary cooling to the finish-rolled steel strip; and coiling the secondary-cooled steel strip. The primary cooling is conducted at cooling speeds of 120° C./sec or more down to the temperatures of from 500 to 800° C. The secondary cooling is conducted at cooling speeds of less than 60° C./sec.

[0001] This application is a continuation application of Internationalapplication PCT/JP00/06639 (not published in English) filed Sep. 27,2000.

FIELD OF THE INVENTION

[0002] The present invention relates to a steel sheet such as hot-rolledsteel sheets and cold-rolled steel sheets, and to a method formanufacturing the same.

BACKGROUND OF THE INVENTION

[0003] Steel sheets such as hot-rolled steel sheets and cold-rolledsteel sheets are used in wide fields including automobiles, householdelectric appliances, and industrial machines. Since these steel sheetsare subjected to some processing before use, they are requested to havevarious kinds of workability.

[0004] Recently, the request of manufacturers of automobiles, householdelectric appliances, industrial machines, and the like relating torationalization becomes severer than ever, particularly in the requestfor improvement in production yield. To cope with the requirement, thematerials thereof are requested to have particularly high homogeneityand high workability level.

[0005] Regarding the workability requested to the hot-rolled steelsheets and cold-rolled steel sheets, high tension materials (hightensile strength hot-rolled steel sheets) having strengths of 340 MPa orhigher class and for the uses other than deep drawing, for example, arerequired to have high stretch flanging performance during burring. Thecold-rolled steel sheets having strengths of 440 MPa or lower and forthe drawing uses are requested to have high r value and high breakingelongation.

[0006] In recent years, the quality requirement of the consumers to thesteel sheets has continuously been increasing, so that not only furtherimprovement in the above-described workability but also homogeneity inmechanical properties in coiled products are strongly requested.

[0007] Responding to these requirements of consumers, several measureshave been studied. For example, in view of the homogeneity of materialquality, JP-A-9-241742, (the term “JP-A” referred herein signifies“Unexamined Japanese Patent Publication”), discloses a method forimproving the homogeneity of mechanical properties in a hot-rolled coilby adopting continuous hot-rolling. The method is a technology that usesa process of continuous hot-rolling to improve the material quality ofthe rolled steel sheet at front end thereof and at rear end thereof, andto eliminate the dispersion in material quality within a coil.

[0008] As for the improvement in workability of high tension materials,JP-B-61-15929 and JP-B-63-67524, (the term “JP-B-” referred hereinsignifies “Examined Japanese Patent Publication”), disclose a method toimprove the workability of high tension hot-rolled steel sheet bycontrolling the cooling speed after the hot-rolling and controlling thecoiling temperature.

[0009] For the improvement in workability of IF steels(Interstitial-Free Steels) JP-A-5-112831 discloses a method to applystrong drafting during hot-rolling and to apply rapid cooling. Thetechnology intends to improve the r value of cold-rolled steel sheet byapplying final reduction in thickness of hot-rolling to 30% or more andby applying rapid cooling immediately after completed the rolling, thusreducing the grain size in the hot-rolled steel sheet.

[0010] All the above-described technologies, however, could not obtainsteel sheet that is superior in both the workability and the homogeneityin mechanical properties. For example, the material properties(determined at center portion of coil width) obtained by the technologydescribed in JP-A-9-241742 aiming at elimination of dispersion ofmaterial quality in a coil gave variations of tensile strength (TS) inan approximate range of from 4.5 to 6.3 kg/mm² for the steel sheets of30 to 70 K class, which range is not satisfactory for users'requirement.

[0011] The technology described in JP-B-61-15929 aiming at theimprovement in workability of high tension materials improves thebalance of strength and ductility compared with conventional steelsheets, but fails to substantially solve the stretch flangingperformance. Furthermore, the technology cannot improve the surfacedefects. Similarly, the high tension hot-rolled steel sheetsmanufactured by the method of JP-B-63-67524 cannot substantially solvethe stretch flanging performance, though the breaking elongation and thetoughness of steel sheets are improved.

[0012] Also the method described in JP-A-5-112831 aiming at theimprovement in workability of IF steels cannot reduce the dispersion ofmaterial quantity to a satisfactory level. That is, according to thedescription of Examples of JP-A-5-112831, the average cooling speedimmediately after the rolling, which average cooling speed is a featureof the invention, is in a range of from 90 to 105° C./sec during 1second after starting the cooling, and from 65 to 80° C./sec during 3seconds after starting the cooling. With that level of cooling speed,however, it was found that, under the hot-rolling condition incommercial apparatuses, the grains in the steel sheet, particularlythose in rolling top portion, cannot be refined.

[0013] The cause is presumably that the cooling cannot be startedimmediately after completed the finish-rolling, and there needs a timeto start cooling. Since the cooling unit cannot be installed at directlyadjacent to the exit of the final rolling stand owing to the necessityof installing finish thermometer and instruments to the exit of thefinal stand of finish-rolling mill, the cooling cannot be performedwithin, for example, 0.1 second after the completion of thefinish-rolling. Particularly at the rolling top portion, high speedtravel is not available and the rolling speed is slow, which results inlong time before starting the cooling. Thus, the cooling at a coolingspeed described in the patent disclosure cannot prevent the formation ofcoarse austenitic grains.

[0014] As described above, the top portion of the steel strip after thehot-rolling is difficult to be rapidly cooled, thus the grains cannot befully reduced in their size, which fails to obtain superior mechanicalproperties and homogeneity thereof. Increased reduction in thickness inthe final pass of hot-rolling is favorable for reducing the size ofaustenitic grains. However, increase of the reduction in thickness to30% or more as in the technology described in JP-A-5-112831 is difficultto be actually implemented because the insufficient shape of steel sheetlikely occurs.

[0015] The automobile industry has a strong need of weight reduction.Accordingly, the use rate of high strength steel sheets has beenincreased. To this point, the high tension materials are inferior inworkability to the mild materials of 270 MPa class, thus there occurproblems of production yield (cracks generated during press-working) andof quality dispersion. Consequently, the improvement in workabilitywhich is a basic characteristic of material quality is requested.

[0016] Regarding the workability, high tension materials having 340 MPaor higher tensile strength, for example, are requested for hot-rolledsteel sheets and cold-rolled steel sheets to have high stretch flangingperformance during burring. In addition, in recent years, the automobileapplication is requested to satisfy the collision safety as one of thecritical characteristics, thus the steel sheets are requested to haveexcellent shock resistance (high shock absorption energy as anevaluation item of collision safety).

[0017] As for the improvement in workability of high tension materials,there is a prior art, Japanese Patent No. 2555436. According to thedisclosure of the patent, a Ti base precipitation hardening steel isprocessed at cooling speeds of from 30 to 150° C./sec after thefinish-rolling, at coiling temperatures of from 250 to 540° C., thusimproving the stretch flanging performance of high tension steels of 50to 60 K class utilizing the formed (ferrite+bainite) structure. However,the cooling speeds of from 30 to 150° C./sec after the finish-rollingcannot be said to substantially improve the stretch flangingperformance, and, there is a problem of low breaking elongation owing tothe low temperature level of coiling.

[0018] JP-B-7-56053 discloses a method to improve the stretch flangingperformance of hot dip zinc-coated steel sheets as the substrate ofhot-rolling sheets using (ferrite+pearlite) steels of 45 to 50 K classapplying cooling speeds of 10° C./sec or more (Examples gave max. 95°C./sec) after the hot-rolling finishing. The cooling speed is, however,95° C./sec at the maximum, and substantial improvement in the stretchflanging performance cannot be attained.

[0019] JP-A-4-88125 discloses a method to improve the stretch flangingperformance of the high tensile materials of 50 to 70 K class using(ferrite+pearlite) steels with the addition of 0.0005 to 0.0050% Ca,applying hot-rolling at high temperatures of (Ar₃ transformation point+60 to 950° C.), and applying cooling within 3 seconds after thehot-rolling at cooling speeds of 50° C./sec or more, preferably 150°C./sec or less, then the cooling is stopped at temperatures of from 410to 620° C. depending on the composition of the steel, followed by aircooling and coiling at 350 to 500° C. of coiling temperatures. Since,however, slight amount of addition of Ca requires an RH degassing stepin the steel making stage, the steel making cost increases. Furthermore,even with the cooling condition after the hot-rolling, which cooling isa feature of the technology, the stretch flanging performance cannot bedrastically improved. In addition, low coiling temperature results inlow breaking elongation.

[0020] As described above, all these prior art technologies cannotattain satisfactory characteristics of stretch flanging performance andbreaking elongation, and furthermore, they did not describe theimprovement in the shock resistance.

[0021] As for the manufacturing of high tension steel sheets, there aremethods to secure strength without adding large amount of alloyingcomponents: the method to strengthen the cooling after rolling; and themethod to reduce grain size. The latter method particularly improves notonly the strength but also the toughness, so that there are manyproposals of the method, including JP-A-58-123823.

[0022] JP-A-61-73829 discloses a method that combines the method tostrengthen the cooling after rolling with the method to reduce grainsize, and the feature of the method is to apply rapid cooling to thesteel sheet, which was once prepared to fine microstructure under anadjustment of rolling condition, for further reducing the grain size.That is, the rapid cooling is given to a state that slight amount offerritic grains were generated during or immediately after the rolling,thus to finely divide the transformed structure using the ferrite tocreate very fine microstructure, which gives steel sheet having highstrength and high toughness.

[0023] The method, however, absolutely requires the precipitation offerrite during or immediately after the rolling owing to the lowtemperature rolling. Therefore, there are problems of, when the rollingfinishing temperature and the temperature to stop cooling varied in therolling width direction or in the rolling longitudinal direction, thestrength varies even in the same composition steels and in a coil, whichfails to attain specified strength.

[0024] As described above, since the prior art intends to refine thegrains by rolling followed by rapid cooling the microscopic structure ofthe steel sheets to secure high strength and high toughness. Owing tothe method, the prior art likely induces unstable characteristics underthe variations in manufacturing conditions.

DISCLOSURE OF THE INVENTION

[0025] First, it is an object of the present invention to provide amethod for manufacturing steel sheet that is applicable forpress-working requiring strict dimensional accuracy, provides superiorworkability including stretch flanging performance, gives uniformmechanical properties and various levels of characteristics, and givesexcellent sheet shape property.

[0026] To attain the object, the present invention provides a method formanufacturing steel sheet comprising the steps of: forming a sheet bar;forming a steel strip; applying primary cooling and secondary cooling tothe steel strip; and coiling the cooled steel strip.

[0027] The step of forming the sheet bar comprises rough-rolling acontinuously cast slab containing 0.8% or less C by weight.

[0028] The step of forming the steel strip comprises finish-rolling thesheet bar at finishing temperatures of not less than (Ar₃ transformationpoint −20° C.).

[0029] The step of cooling the steel strip comprises cooling thefinish-rolled steel strip at cooling speeds of higher than 120° C./secdown to temperatures of from 500 to 800° C.

[0030] The step of coiling the cooled steel strip comprises coiling thesecondary-cooled steel strip at temperatures of from 400 to 750° C.

[0031] In the method for manufacturing steel sheet, when a sheet bar isformed by rough-rolling a continuously cast slab containing more than0.8% and not more than 1% C by weight, the sheet bar is finish-rolled atfinishing temperatures of not less than (Acm transformation point −20°C.).

[0032] Secondly, it is an object of the present invention to provide amethod for manufacturing steel sheet that induces less failures informing to a product shape, is possible to conduct product layout on acoil at high yield, has superior workability of stretch flangingperformance and breaking elongation, has high shock resistance, andgives excellent tensile strength as high as 340 MPa or more.

[0033] To attain the object, the present invention provides a method formanufacturing steel sheet comprising the steps of: forming a slab;forming a hot-rolled steel sheet; applying primary cooling and secondarycooling to the hot-rolled steel sheet; and coiling the cooled steelsheet.

[0034] The step of forming the slab comprises continuous casting to givetreatment for reducing segregation to manufacture the slab consistingessentially of 0.05 to 0.14% C, 0.5% or less Si, 0.5 to 2.5% Mn, 0.05%or less P, 0.1% or less S, 0.005% or less O, and less than 0.0005% Ca,by weight.

[0035] The step of forming the hot-rolled steel sheet compriseshot-rolling the slab at finishing temperature of finish-rolling not lessthan Ar₃ transformation point.

[0036] The primary cooling step comprises cooling the hot-rolled steelsheet starting the primary cooling within 2 seconds after thehot-rolling to temperatures of from 600 to 750° C. at cooling speeds offrom 100 to 2,000° C./sec.

[0037] The secondary cooling step comprises cooling the primary-cooledsteel sheet starting the secondary cooling to the above-describedtemperature range at cooling speeds of less than 50° C./sec. Thesecondary-cooled steel sheet is coiled at temperatures of from 450 to650° C.

[0038] Thirdly, it is an object of the present invention to provide amethod for manufacturing steel sheet that provides wanted strengthcharacteristics stably.

[0039] To attain the object, the present invention provides a method formanufacturing steel sheet comprising hot-rolling step and cooling step.

[0040] The step of hot-rolling comprises hot-rolling a steel consistingessentially of 0.03 to 0.12% C, 1% or less Si, 5 to 2% Mn, 0.02% or lessP, 0.01% or less S, at least one element selected from the groupconsisting of 0.005 to 0.1% Nb, 0.005 to 0.1% V, and 0.005 to 0.1% Ti,by weight, at temperatures of 1,070° C. or below to accumulatedreductions in thickness of 30% or more.

[0041] The step of hot-rolling may be carried out on a steel consistingessentially of 0.03 to 0.12% C, 1% or less Si, 0.5 to 2% Mn, 0.02% orless P, 0.01% or less S, and 0.05 to 0.5% Mo, by weight, at temperaturesof 1,070° C. or below to accumulated reductions in thickness of 30% ormore.

[0042] The step of cooling comprises cooling steel sheet within 6seconds after the completion of the rolling to temperatures higher than500° C. and not higher than 700° C. at average cooling speeds of notless than 80° C./sec.

BRIEF DESCRIPTION OF THE DRAWING

[0043]FIG. 1 shows the influence of the time to start the primarycooling on the mechanical properties according to the Preferredembodiment 2.

[0044]FIG. 2 shows the relation between the tensile strength and thebore expanding rate according to the Preferred embodiment 2.

[0045]FIG. 3 shows the influence of the temperature to stop the rapidcooling (primary cooling) on the strength characteristics (TS, YS)according to the Preferred embodiment 3.

[0046]FIG. 4 shows the influence of the temperature to stop the rapidcooling (primary cooling) on the strength characteristic (EI) accordingto the Preferred embodiment 3.

[0047]FIG. 5 shows the influence of the temperature to stop the rapidcooling (primary cooling) on the strength characteristics (TS, EI)according to the Preferred embodiment 3.

[0048]FIG. 6 shows the influence of the temperature to stop the rapidcooling (primary cooling) on the strength characteristic (YR) accordingto the Preferred embodiment 3.

[0049]FIG. 7 shows the influence of the temperature to stop the rapidcooling (primary cooling) on the toughness according to the Preferredembodiment 3.

PREFERRED EMBODIMENT FOR CARRYING OUT THE INVENTION

[0050] Preferred Embodiment 1

[0051] The method for manufacturing steel sheet according to thePreferred embodiment 1 comprises the steps of: forming a sheet bar byrough-rolling a continuous cast slab containing 0.8% or less C byweight; forming a steel strip by finish-rolling the sheet bar atfinishing temperatures of finish-rolling of not less than (Ar₃transformation point −20° C.); rapid cooling the steel strip after thefinish-rolling down to temperatures of from 500 to 800° C. at coolingspeeds of higher than 120° C./sec; and coiling the steel strip after therapid cooling at coiling temperatures of from 400 to 750° C.

[0052] In the manufacturing method, the continuously cast slab may beprepared by continuously casting a steel consisting essentially of 0.8%or less C, 2.5% or less Si, and 3.0% or less Mn, by weight. Furthermore,the continuously cast slab may be prepared by continuously casting asteel consisting essentially of 0.8% or less C, 2.5% or less Si, 3.0% orless Mn, and 0.01 to 0.2% at least one element selected from the groupconsisting of Ti, Nb, V, Mo, Zr, and Cr, by weight. Furthermore, thecontinuously cast slab may be prepared by continuously casting a steelconsisting essentially of 0.8% or less C, 2.5% or less Si, 3.0% or lessMn, and 0.005% or less at least one of Ca and B, by weight.

[0053] In these manufacturing methods, the continuously cast slab may beprepared by continuously casting a steel consisting essentially of 0.8%or less C, 2.5% or less Si, 3.0% or less Mn, 0.01 to 0.2% at least oneelement selected from the group consisting of Ti, Nb, V, Mo, Zr, and Cr,and 0.005% or less at least one of Ca and B, by weight.

[0054] In these manufacturing methods, the C content may be specified toa range of from more than 0.8% and not more than 1.0% by weight, insteadof 0.8% or less, and the finishing temperature may be specified to (Acmtransformation point −20° C.) instead of (Ar₃ transformation point −20°C.), while adopting the same conditions for other variables.

[0055] The above-described aspects of the invention have been derivedduring the keen studies to solve the above-described problems. In thecourse of the studies, the inventors of the present invention found thatthe workability of steel sheets and the homogeneity of mechanicalproperties thereof are significantly influenced by the time betweenimmediately after the rolling and the start of cooling and by thecooling speed. After investigating these variables, the inventors of thepresent invention have successfully manufactured steel sheet havingexcellent workability and homogeneous mechanical properties, allowinghigh yield product layout on a coil, from the standpoint of useconditions at manufacturers of automobiles, household electricappliances, industrial machines, and the like. The detail of themanufacturing method according to the present invention is described inthe following. First, the chemical composition of steel is described.

[0056] C: 1% or Less (by Weight: Hereinafter the Same Unit is Applied)

[0057] Carbon is an additive element to ensure the strength of steel.Excessive addition, however, results in significant degradation inworkability. That is, more than 1% C content induces degradation inworkability. Accordingly, the C content is specified to 1% or less.

[0058] Si: 2.5% or Less

[0059] Silicon is an element to strengthen solid solution. If, however,the Si content exceeds 2.5%, the surface properties degrade.Consequently, the Si content is preferably 2.5% or less.

[0060] Mn: 3% or Less

[0061] Manganese improves toughness of the steel sheet and has functionto strengthen solid solution. However, Mn is an element that gives badinfluence on workability. If the Mn content exceeds 3%, the strengthincreases to significantly degrade the workability. Therefore, the Mncontent is preferably 3% or less.

[0062] P: 0.2% or Less

[0063] Phosphorus is an element that has a function to strengthen solidsolution. If, however, the P content exceeds 0.2%, grain boundarybrittleness caused from grain boundary segregation likely occurs.Accordingly, the P content is preferably 0.2% or less.

[0064] S: 0.05% or Less

[0065] Sulfur is an impurity element, and the S content is preferablyminimized. If the S content exceeds 0.05%, fine sulfide precipitationincreases to degrade the workability. Consequently, the S content ispreferably 0.05% or less.

[0066] N: 0.02% or less

[0067] Less amount of N reduces further the necessary adding amount ofcarbo-nitride forming elements, which are described later, to improveeconomy. If the N content exceeds 0.02%, the degradation in workabilityof steel sheet unavoidably occurs even when carbo-nitride formingelements are added to fix N. Therefore, the N content is preferably0.02% or less.

[0068] O: 0.005% or less

[0069] Oxygen content is required to be controlled to suppress crackgeneration on the surface of slab or below the surface layer of slabduring continuous casting. If the O content exceeds 0.005%, the crackgeneration on slab becomes significant, and the workability which is anaim of the present invention also degrades. Accordingly, the O contentis preferably 0.005% or less.

[0070] At Least One Element Selected from the Group Consisting of Ti,Nb, V, Mo, Zr and Cr: 0.01 to 0.2%

[0071] Adding to the above-described chemical components, necessaryamounts of Ti, Nb, V, Mo, Zr, Cr are added to adjust the strength or toimprove the non-aging effect (and to improve the deep drawingperformance) utilizing the reduction in solid solution C and N resultedfrom the formation of carbo-nitrides. The sum of added these elementsless than 0.01% gives no effect, and more than 0.2% degrades theworkability such as ductility and deep drawing performance.Consequently, if Ti, Nb, V, Mo, Zr, Cr are added, the sum of theseelements are specified to a range of from 0.01 to 0.2%.

[0072] At Least One Element Element Selected from the Group Consistingof Ca and B: 0.005% or Less

[0073] According to the present invention, Ca and B are effectiveelements to improve the workability of steel sheet, so these elementsare preferably to be added. If, however, the sum of the Ca and Bcontents exceeds 0.005%, the deep drawing performance is degraded.Therefore, if Ca and/or B are added, the sum of the added contents isspecified to 0.005% or less.

[0074] Next, the manufacturing conditions according to the presentinvention are described below.

[0075] Finishing Temperature (for the Case of C≦0.8%): (Ar₃Transformation Point −20° C.) or Above

[0076] When the C content is 0.8% or less, if the finishing temperatureis below the (Ar₃ transformation point −20° C.), the ferritetransformation proceeds in a part of the steel microstructure, resultingin working on the ferritic grains, which leads to unfavorable materialquality such as enhanced nonhomogeneous material quality and intraplaneanisotropy. Therefore, according to the present invention, when the Ccontent is 0.8% or less, the finish-rolling is applied at finishingtemperatures of (Ar₃ transformation point −20° C.) or above. Thefinish-rolling assures the homogeneous structure and the reduced grainsize in succeeding steps, thus improves the workability such as thebalance of strength and ductility, the stretch flanging performance, andincreases the r value in a cold-rolled steel sheet.

[0077] Finishing Temperature (for the Case of C>0.8%): (AcmTransformation Point −20° C.) or Above

[0078] When the C content exceeds 0.8%, if the finishing temperature isbelow the (Acm transformation point −20° C.), the cementite which isprecipitated at austenitic grain boundaries increases to fail to formhomogeneous pearlite structure, which results in nonhomogeneousmicrostructure. Thus, according to the present invention, when the Ccontent exceeds 0.8%, the finish-rolling is applied at finishingtemperature of (Ar₃ transformation point −20° C.) or above. Thefinish-rolling assures the homogeneous microstructure and the reducedgrain size in succeeding steps, thus improves the workability such asthe quenching performance, the spheroidizing rate in cold-rolled steelsheet, and the stretch flanging performance.

[0079] Cooling after Rolled: Rapid Cooling at Cooling Speed>120° C./sec

[0080] According to the present invention, rapid cooling after rolled isnecessary to establish fine structure of ferritic grains, pearlite andthe like after the transformation and to uniformize the materialquality. If the cooling is gradual cooling, the microstructure becomescoarse one, and in a high C steel, homogeneous pearlite structure cannotbe attained to result in nonhomogeneous microstructure. If the coolingspeed is 120° C./sec or less, the ferritic grains and the structure ofpearlite and the like generated from transformation become coarse, andin a hypereutectoid steel, cementite precipitates to result innonhomogeneous microstructure.

[0081] End Temperature of Cooling: 500 to 800° C.

[0082] If rapid cooling is given down to below 500° C., the difference(margin) between the cooling temperature and the coiling temperaturebecomes less, which makes temperature homogenization difficult.Furthermore, additional cooling unit for the rapid cooling becomesnecessary, which increases the investment cost. To the contrary, if theend temperature of cooling exceeds 800° C., only a part of themicrostructure is transformed to give nonhomogeneous one, thus themicrostructure becomes coarse during the cooling (slow cooling)accompanied with the temperature adjustment during the succeedingcoiling step.

[0083] Accordingly, after the rolling, when the steel strip is subjectedto primary cooling at cooling speeds of higher than 120° C./sec down tothe temperatures of from 500 to 800° C., the ferritic grains and theprecipitates of pearlite and the like become fine in their size afterthe transformation, which improves the workability. The upper limit ofthe cooling speed is not specifically specified. From the viewpoint ofindustrial applicability, however, the upper limit of the cooling speedis 2,000° C./sec.

[0084] Coiling Temperature: 400 to 750° C.

[0085] After the secondary cooling, the steel strip is required to becoiled at coiling temperatures of from 400 to 750° C. The reason is thatless than 400° C. of coiling temperature induces the formation of lowtemperature transformation phase, and that above 750° C. of coilingtemperature induces formation of coarse microstructure of grains and thelike to degrade the workability.

[0086] The basic manufacturing conditions according to the presentinvention are described above. The following-described manufacturingconditions may further be applied, at need.

[0087] Treatment in the Course of from Continuous Casting toRough-Rolling: Direct Rolling or Warm Feeding

[0088] The continuously cast slab may be roughly-rolled either by directhot-rolling or by reheating, before cooling to room temperature, totemperatures of 1,200° C. or below by feeding at warm state into aheating furnace. According to the present invention, the continuouslycast slab is not cooled to room temperature but started therough-rolling with direct-rolling in as-cast state, or is reheated totemperatures of 1,200° C. or below, followed by starting rough-rolling.As a result, the temperature of slab before rolling becomes uniform andthe mechanical properties in a coil becomes further homogeneous.

[0089] Treatment in the Course of from Immediately Before theFinish-Rolling to During the Rolling: Induction Heating

[0090] The material to be rolled may be heated by an induction heatingunit immediately before the finish-rolling or during the finish-rolling.According to the present invention, the temperature of the materialduring rolling becomes more uniform and the mechanical properties in acoil become more homogeneous.

[0091] Time to Start the Rapid Cooling: more than 0.1 Second and Lessthan 1.0 Second

[0092] After the finish-rolling, the rapid cooling can start within aperiod ranging from more than 0.1 second and less than 1.0 second.According to the present invention, the ferritic grains and theprecipitates of pearlite and the like are refined after thetransformation, which further improves the workability.

[0093] Treatment After Coiling: Cold-Rolling to Annealing

[0094] The steel sheet manufactured by the above-described method mayfurther be subjected to cold-rolling and annealing. According to thepresent invention, the material properties and structure of thehot-rolled coil are homogeneous, the annealing after the cold-rollingprovides a cold-rolled steel sheet that has excellent workability andhomogeneity of mechanical properties.

[0095] Thus, according to the present invention, the reduction invariations of temperature in a coil allows to manufacture a steel sheetin which the variations (maximum and minimum values) of tensile strengthof the hot-rolled steel strip in the width direction and in thelongitudinal direction thereof are within ±8% of the average of thetensile strength in a coil. The steel sheet having that small variationsgives small variations of press-workability (such as spring back duringbending) in a coil. That type of steel sheet contributes to the productyield and shape accuracy after the press-working at users' shops. Thatis, the steel sheet has excellent performance as the material.

[0096] On carrying out the present invention, the steel composition isnot specifically limited, and common existing compositions of hot-rolledsteel sheets and cold-rolled steel sheet that have variouscharacteristics may be applied. That is, simple carbon steel sheets orsteel sheets containing special elements such as Ti, Nb, V: Mo, Zr, Ca,B are also applicable. According to the present invention, the additionof 0.02 to 2% Cu and the addition of 0.01% or less Sn are allowable.Within that range of Cu and Sn contents, these elements do not degradethe effect of the present invention.

[0097] When a continuously cast slab is not cooled to room temperaturebut started rough-rolling after heated to 1,200° C. or lowertemperature, the temperature of slab before the rolling can beuniformized, thus the mechanical properties in a coil can further behomogenized. After the continuously cast slab is roughly-rolled, whenthe sheet bar immediately before the finish-rolling is, or when thematerial during the finish-rolling is heated by an induction heatingunit, the temperature of the material during rolling can further beuniformized, and the mechanical properties in a coil can further behomogenized.

[0098] In the finish-rolling, the reduction in thickness in the finalpass is preferably set to 8% or more and less than 30%. The reason isthat full reduction of austenitic grain size preferably requires 8% orhigher reduction in thickness, and that sustaining good shape of steelsheet preferably requires less than 30% reduction in thickness. From thepoint of size reduction in the hot-rolled steel sheet, it is preferablethat the reduction in thickness at each rolling pass is set to higherthan 10%.

[0099] As for the finishing temperature, when the C content is 0.8% orless, if the finish-rolling is conducted at temperatures of from (Ar₃transformation point −20° C.) to (Ar₃ transformation point +50° C.), thegrains immediately after the finish-rolling, or before the runout tablecooling, can be refined. By adopting the finishing temperature of (Ar₃transformation point +50° C.) or less, the formation of coarseaustenitic grains is prevented, and the reduction in ferritic grain sizeafter rolling becomes easy. As a result, the refinement of grains insucceeding steps can be attained, thus improving the workability such asthe balance of strength and ductility, the stretch flanging performance,and high r value in cold-rolled steel sheet.

[0100] When the C content exceeds 0.8%, if the finish-rolling is givenat temperatures of from (Acm transformation point −20° C.) to (Acmtransformation point +100° C.), while adopting the other conditions sameas those in the case of 0.8% or less C, the steel sheet having excellentworkability and homogeneous mechanical properties can be obtained. Byadopting the finishing temperature to (Acm transformation point +100°C.) or below, the formation of coarse austenitic grains is prevented andthe formation of fine pearlite colony after the rolling can be attained.

[0101] When the finishing temperature differs depending on the positionson a material being rolled in width direction and in longitudinaldirection, and when the difference therebetween becomes significant, thestructure of steel strip becomes nonhomogeneous. Thus, the difference infinishing temperature is preferably maintained to a small level. If thefinish-rolling is conducted so as the finishing temperature differencein a material being rolled to fall in 50° C. range, the microstructureof steel strip immediately after the finish-rolling becomes homogeneous,and the homogeneity of the mechanical properties after coiled isassured. As a result, the difference in microstructure and materialproperties of final products can be neglected. Therefore, the differencein finishing temperature in a material being rolled is preferably 50° C.or less.

[0102] After the rolling, to establish fine microstructure of ferriticgrains and pearlite and the like and to establish homogeneous materialquality, the cooling after the rolling is preferably in combination ofrapid cooling and slow cooling. By applying slow cooling after the rapidcooling, the local irregularity of end temperature of cooling isreduced, and the variations in absolute values of end temperature ofcooling become less, so that the variations in material quality level isdiminished. Above-described rapid cooling and slow cooling arehereinafter referred to the primary cooling and the secondary cooling,respectively.

[0103] Primary cooling to temperatures of from 500 to 800° C. at coolingspeeds exceeding 120° C./sec improves the workability through therefinement of ferritic grains and of pearlite structure after thetransformation. At that moment, extremely superior workability isattained by applying the cooling at cooling speeds of 200° C./sec ormore, more preferably of 400° C./sec or more, from the viewpoint ofreduction in size of ferritic grains and of pearlite structure. Althoughthe upper limit of the cooling speed is not specifically specified,industrial application has a limit of approximately 2,000° C./sec.

[0104] To reduce the dispersion of material properties of hot-rolledsteel strip to further preferable level, it is preferred for thetemperature to stop the rapid cooling to regulate within the range ofthe present invention and for the temperature variations (maximumvalue-minimum value) in the width direction and in the longitudinaldirection of coil after the rapid cooling to regulate within 60° C.

[0105] More preferably, by regulating the variations of tensile strengthto within ±4%, the above-described performance at users site can besignificantly improved. In that case, by regulating the variations oftemperature to stop the rapid cooling to within 40° C., the variationsin the material quality can be minimized.

[0106] To further reduce the variations of tensile strength to within±2%, the above-given variations of temperature to stop the rapid coolingmay be regulated to within 20° C. The reduction in variations ofmaterial quality can be determined from the relation between thevariations of these temperatures and the tensile strength. Thetemperature in the coil width direction according to the presentinvention covers the range of coil width except for the 30 mm area fromeach of the edges thereof.

[0107] As for the performance of the rapid cooling (primary cooling),the variations in temperature after the rapid cooling can be reduced byapplying cooling with a heat transfer coefficient of 2,000 kcal/m²h ° C.Preferred heat transfer coefficients to reduce the variations oftemperature are 5,000 kcal/m²hr ° C. or more, further preferably 8,000kcal/m²h ° C. or more.

[0108] For the primary cooling, if the cooling starts within a period offrom more than 0.1 second to less than 1.0 second after thefinish-rolling, the post-transformation ferritic grains and precipitatessuch as pearlite can be refined, thus the workability can further beimproved. To attain more preferable level of dispersion of materialquality in hot-rolled steel strip, the time to start cooling ispreferably longer than 0.5 second after the finish-rolling.

[0109] After the primary cooling, preferably slow cooling (secondarycooling) is applied for adjusting the coiling temperature. Inparticular, when the cooling speed of the secondary cooling is less than60° C./sec, accurate temperature control is available, thus the endtemperature of cooling, or the temperature of coiling, becomes uniform.As a result, the structure of coil after the coiling becomes furtherhomogeneous, so that it is preferable to give the secondary cooling tothe steel strip at cooling speeds of less than 60° C./sec forhomogenizing the mechanical properties in a coil.

[0110] After the secondary cooling, the steel strip is necessary to becoiled at temperatures of from 400 to 750° C. The reason is that thecoiling temperatures of less than 400° C. induces the formation of lowtemperature transformed phase, and that the coiling temperature ofhigher than 750° C. induces formation of coarse structure of grains orthe like to degrade the workability. As for the coiling temperature ofhigh C materials, the coiling temperature is preferred to be applied at450° C. or more to prevent the formation of low temperature transformedphase. From the viewpoint of homogenization of the material quality offinal products, it is preferred to regulate the difference in coilingtemperature in a coil to 80° C. or less.

[0111] The present invention can also be applied to the direct rollingprocess in which a continuously cast slab is directly hot-rolled withoutpassing through a heating furnace. The present invention is alsoeffective to the continuous rolling process that uses a coil box and thelike. When the material being rolled is heated by an induction heatingunit immediately before the finish-rolling or during the finish-rolling,the present invention is also effective when edge heating is applied.

[0112] Annealing thus obtained hot-rolled coil after the cold-rolledprovides cold-rolled steel sheet having both excellent workability andexcellent homogenization of mechanical properties. In that case, theannealing is preferably applied by continuous annealing to assurehomogeneity of the mechanical properties.

EXAMPLE 1

[0113] Steels Nos. 1 through 7 having the chemical compositions given inTable 1 were prepared by melting. All these steels have the chemicalcompositions within the range of the present invention. The steels wererolled under the hot-rolling conditions given in Table 2 to formrespective hot-rolled coils Nos. 1 through 14, each having a thicknessof 3 mm. The heat transfer coefficients in the rapid cooling (primarycooling) in Example 1 were 3,000 to 4,000 kcal/m²h ° C.

[0114] Tension testing specimens were prepared by cutting at 5 positionson each of the hot-rolled coil in the longitudinal direction thereof. Oneach specimen, average tensile strength (TS), total elongation (El),dispersion in tensile strength (ΔTS), and dispersion in total elongation(ΔEl) were determined. For a part of the hot-rolled coils, boreexpanding rate (γ) and dispersion in bore expanding rate (γλ) weredetermined. Furthermore, for the hot-rolled coils Nos. 4 through 7 andNos. 11 through 13, cold-rolling was applied after pickling to a sheetthickness of 0.8 mm, followed by applying continuous annealing, then ther value was determined to evaluate the deep drawing performance. Table 3shows the result of determination of these mechanical properties of thehot-rolled coils and the cold-rolled and annealed sheets.

[0115] As clearly seen by comparing the steel sheets Nos. 1 through 8 ofthe Examples of the present invention with the steel sheets Nos. 9through 14 of the Comparative Examples, having respective chemicalcompositions, the dispersions of mechanical properties, ΔTS, ΔEl, andΔλ, were smaller in the Examples of the present invention than those inthe Comparative Examples, for all the chemical compositions tested. Tothe contrary, the steel sheets Nos. 9 through 14 of the ComparativeExamples failed to satisfy one or more of the manufacturing conditionsspecified by the present invention, giving inferior homogeneity in themechanical properties or inferior workability to the steel sheets Nos. 1through 8 of the Examples of the present invention having the samechemical composition to the Comparative Example steels.

EXAMPLE 2

[0116] Steels Nos. 1 through 7 having the chemical compositions given inTable 1 were rolled under the hot-rolling conditions given in Table 4 toform respective hot-rolled coils Nos. 15 through 28, each having athickness of 3 mm. The heat transfer coefficients in the primary coolingwere 12,000 kcal/m²hr ° C. for the steels Nos. 15 through 22 of theExamples of the present invention, and 1,000 kcal/m²hr ° C. for thesteels Nos. 23 through 28 of the Comparative Examples.

[0117] Similar with the Example 1, the dispersion in mechanicalproperties in the width direction and in the longitudinal direction ofthese hot-rolled coils were determined. Furthermore, the hot-rolledcoils Nos. 18 through 22 and Nos. 26 through 28 were cold-rolled afterthe pickling to a thickness of 0.8 mm, followed by applying continuousannealing, then the r value was determined to evaluate the deep drawingperformance. Table 5 shows the result of determination of thesemechanical properties of the hot-rolled coils and the cold-rolled andannealed sheets.

[0118] In the table, ΔTS and ΔEl indicate the half value of thedifference between the maximum value and the minimum value of TS and El,respectively. To determine the tensile characteristics, specimens weresampled from the coil excluding the portions of 30 mm from each edge inthe coil width and of 5 m from each end in the coil length. The averageof all the determined values was adopted as the intra-coil average.

[0119] As clearly seen by comparing the-steel sheets Nos. 15 through 22of the Examples of the present invention with the steel sheets Nos. 23through 28 of the Comparative Examples, having respective chemicalcompositions, the dispersions of mechanical properties, ΔTS and ΔEl,were smaller in the Examples of the present invention than those in theComparative Examples, for all the chemical compositions tested. To thecontrary, the steel sheets Nos. 23 through 28 of the ComparativeExamples failed to satisfy one or more of the manufacturing conditionsspecified by the present invention, giving inferior homogeneity in themechanical properties or inferior workability to the steel sheets Nos.15 through 22 of the Examples of the present invention having the samechemical composition to the Comparative Example steels.

[0120] According to the present invention, the variations of temperatureto stop the rapid cooling (primary cooling) in a coil are smaller thanthose in the conventional laminar cooling in prior art, and thevariations in mechanical properties are reduced to further preferablelevel. The cooling method according to the present invention is theperforated ejection type providing high heat transfer coefficient.

[0121] As described above, the present invention allows to manufacturesteel sheet that has excellent homogeneity of mechanical properties in acoil, giving high El and λ values of hot-rolled coil and high r valueafter cold-rolled and annealed, and providing excellent workability.TABLE 1 Steel Weight % No. C Si Mn S P O N Ti Nb V Mo Zr B Ca 1 0.8500.24 0.47 0.003 0.017 0.0020 0.0025 — — — — 0.005 — — 2 0.061 0.03 0.710.001 0.012 0.0021 0.0020 — — 0.010 — — — — 3 0.166 0.01 0.70 0.0040.016 0.0022 0.0031 — — — — — — 0.002 4 0.021 0.01 0.22 0.008 0.0160.0018 0.0026 — — — — — 0.0025 — 5 0.0020 0.02 0.21 0.005 0.010 0.00210.0014 0.035 — — 0.010 — 0.0003 — 6 0.0015 0.25 0.65 0.008 0.050 0.00200.0020 0.031 0.015 — — — — — 7 0.0015 0.25 0.65 0.008 0.050 0.00200.0020 0.008 0.023 — — — — —

[0122] TABLE 2 Difference in Finish final end reduction in Endtemperature of temperature Steel Slab heat- thickness rolling of rollingsheet Steel treatment history (%) (° C.) (° C.) 1 1 Casting, then 10(Arcm + 40)˜(Arcm + 60) 20 heating to 1,250° C. 2 2 Casting then hot 10(Ar3 + 20)˜(Ar3 + 45) 25 direct rolling 3 3 Casting then hot 15 (Ar3 +30)˜(Ar3 + 50) 20 direct rolling 4 4 Casting, then 15  (Ar3 + 5)˜(Ar3 +20) 15 heating to 1,200° C. 5 5 Casting, then 15  (Ar3 + 5)˜(Ar3 + 15)10 heating to 1,200° C. 6 6 Casting, then 15     Ar3˜(Ar3 + 10) 10heating to 1,200° C. 7 6 Casting, then 10     Ar3˜(Ar3 + 10) 10 heatingto 1,200° C. 8 7 Casting, then 10     Ar3˜(Ar3 + 10) 10 heating to1,200° C. 9 1 Casting, then 15 (Arcm − 10)˜(Arcm + 50) 60 heating to1,250° C. 10  2 Casting, then hot 15 (Ar3 + 25)˜(Ar3 + 40) 15 directrolling 11  3 Casting, then hot 15 (Ar3 + 25)˜(Ar3 + 50) 25 directrolling 12  4 Casting, then 20     Ar3˜(Ar3 + 20) 20 heating to 1,200°C. 13  5 Casting, then 35  (Ar3 + 5)˜(Ar3 + 15) 10 heating to 1,200° C.14  6 Casting, then 15     Ar3˜(Ar3 + 15) 15 heating to 1,200° C. EndTime to start Primary temperature of Secondary the runout cooling theprimary cooling Coiling Steel table cooling speed cooling speedtemperature sheet Steel (sec) (° C./sec) (° C.) (° C./sec) (° C.) Remark1 1 1.3 200 650 15 600˜625 E 2 2 0.9 205 670 20 590˜620 E 3 3 0.5 160680 25 570˜600 E 4 4 0.3 200 680 10 605˜625 E 5 5 0.2 210 690 20 630˜650E 6 6 0.4 200 680 25 635˜648 E 7 6 1.2 200 680 25 630˜645 E 8 7 1.2 200680 25 625˜650 E 9 1 1.2 190 660 15 595˜620 C 10  2 0.8 200 700 65585˜610 C 11  3 0.5 170 680 25 685˜710 C 12  4 0.3 180 690 60 600˜615 C13  5 0.2 80 700 50 620˜643 C 14  6 1.2 200 670 65 630˜648 C

[0123] TABLE 3 Steel sheet Steel Mechanical properties of hot-rolledsteel sheet Shape of hot-rolled r value after cold- No. No. TS (Mpa) ΔTS (Mpa) El (%) Δ El (%) λ (%) Δ λ (%) steel sheet rolled and annealedRemark 1 1 1018 40 16 3 — — Good — Example 2 2 640 25 25 5 100 20 Good —Example 3 3 505 18 36 6 150 32 Good — Example 4 4 359 12 45 6 — — Good1.6 Example 5 5 284 10 47 5 — — Good 2.7 Example 6 6 355 11 42 4 — —Good 2.7 Example 7 6 350 10 43 4 — — Good 2.5 Example 8 7 355 9 42 4 — —Good 2.6 Example 9 1 1015 70 15 6 — — Good — Comparative example 10 2640 51 23 7 90 35 Good — Comparative example 11 3 457 26 30 9 95 36 Good— Comparative example 12 4 361 22 41 8 — — Good 1.3 Comparative example13 5 280 11 46 6 — — Bad 2.2 Comparative (significant example edge wave)14 6 349 21 42 6 — — Good 2.4 Comparative example

[0124] TABLE 4 Difference in end Time to start End temperature oftemperature of the runout Steel Slab heat-treatment rolling rollingtable cooling sheet Steel history (° C.) (° C.) (sec) 15 1 Casting, thenheating to (Arcm + 45)˜(Arcm + 60) 15 1.3 1,250° C. 16 2 Casting, thenhot direct (Ar3 + 20)˜(Ar3 + 40) 20 0.9 rolling 17 3 Casting, then hotdirect (Ar3 + 30)˜(Ar3 + 45) 15 0.6 rolling 18 4 Casting, then heatingto  (Ar3 + 5)˜(Ar3 + 20) 15 0.6 1,200° C. 19 5 Casting, then heating to (Ar3 + 5)˜(Ar3 + 20) 15 0.6 1,200° C. 20 6 Casting, then heating to    Ar3˜(Ar3 + 15) 15 0.6 1,200° C. 21 6 Casting, then heating to    Ar3˜(Ar3 + 10) 10 1.2 1,200° C. 22 7 Casting, then heating to    Ar3˜(Ar3 + 15) 15 1.2 1,200° C. 23 1 Casting then heating to (Arcm −10)˜(Arcm + 50) 60 1.2 1,250° C. 24 2 Casting, then hot direct (Ar3 +25)˜(Ar3 + 40) 15 0.8 rolling 25 3 Casting, then hot direct (Ar3 +25)˜(Ar3 + 50) 25 0.5 rolling 26 4 Casting, then heating to    Ar3˜(Ar3 + 20) 20 0.3 1,200° C. 27 5 Casting, then heating to (Ar3 +5)˜(Ar3 + 15) 10 0.2 1,200° C. 28 6 Casting, then heating to    Ar3˜(Ar3 + 15) 15 1.2 1,200° C. End Primary temperature of Secondarycooling the primary cooling Coiling Steel speed cooling speedtemperature sheet Steel (° C./sec) (° C.) (° C./sec) (° C.) Remark 15 1430 635˜662 20 600˜620 E 16 2 440 655˜681 20 590˜620 E 17 3 440 665˜69330 575˜600 E 18 4 435 665˜690 10 605˜625 E 19 5 420 678˜702 25 635˜650 E20 6 450 663˜695 25 635˜645 E 21 6 430 667˜696 20 625˜645 E 22 7 420660˜700 25 630˜650 E 23 1 60 630˜700 20 595˜620 C 24 2 50 651˜734 65585˜620 C 25 3 40 635˜724 20 685˜720 C 26 4 50 645˜721 60 600˜625 C 27 550 657˜730 45 620˜653 C 28 6 50 635˜705 60 630˜658 C

[0125] TABLE 5 Steel r value after sheet Steel Mechanical properties ofhot-rolled steel sheet cold-rolled and No. No. TS (Mpa) Δ TS (Mpa) El(%) Δ El (%) annealed Remark 15 1 1015 32 17 2 — Example 16 2 632 17 264 — Example 17 3 500 13 38 5 — Example 18 4 354 8 45 5 1.7 Example 19 5280 7 48 4 2.8 Example 20 6 352 6 43 2 2.7 Example 21 6 351 7 43 2 2.6Example 22 7 353 8 43 2 2.3 Example 23 1 1014 90 13 6 — Comparativeexample 24 2 641 55 23 6 — Comparative example 25 3 458 41 30 8 —Comparative example 26 4 360 32 40 7 1.3 Comparative example 27 5 281 2543 7 2.1 Comparative example 28 6 340 31 41 6 2.2 Comparative example

[0126] Preferred Embodiment 2

[0127] The inventors of the present invention carried out extensivestudies to improve the stretch flanging performance, the breakingelongation, and the shock resistance focusing on high tension steelswhich were manufactured by reheating continuously cast slab followed byhot-rolling thereof or which were manufactured by directly hot-rollingthe continuously cast slab without reheating. Thus, the inventors of thepresent invention found that the stretch flanging performance and thebreaking elongation are influenced by the presence of a banded structureenriched with C, Mn, or the like at center portion of the sheetthickness, and that the improvement in shock resistance becomeseffective when the yield strength of the material is increased to alevel that does not degrade the workability of the material.

[0128] These findings were further investigated to derive the presentinvention. That is, the present invention provides:

[0129] 1. A method for manufacturing steel sheet consisting essentiallyof 0.05 to 0.14% C, 0.5% or less Si, 0.5 to 2.5% Mn, 0.05% or less P,0.01% or less S, 0.005% or less O, and less than 0.0005% Ca, by weight,which method comprises the steps of: (1) forming a slab by continuouscasting conducting treatment to reduce segregation; (2) hot-rolling theslab at end temperatures of finish-rolling of Ar₃ transformation pointor above; (3) starting the primary cooling within 2 seconds aftercompleted the hot-rolling at cooling speeds of from 100 to 2,000° C./secto cool the hot-rolled steel sheet to temperatures of from 600 to 750°C.; (4) applying the secondary cooling after the primary cooling atcooling speeds of less than 50° C./sec, followed by applying coiling tothe secondary cooled hot-rolled steel sheet at temperatures of from 450to 650° C.

[0130] 2. A method for manufacturing steel sheet consisting essentiallyof 0.05 to 0.14% C, 0.5% or less Si, 0.5 to 2.5% Mn, 0.05% or less P,0.01% or less S, 0.005% or less O, and less than 0.0005% Ca, by weight,which method comprises the steps of: (1) forming a slab by continuouscasting conducting treatment to reduce segregation; (2) reheating theslab before applying hot-rolling; (3) hot-rolling the slab at endtemperatures of finish-rolling of Ar₃ transformation point or above; (4)starting the primary cooling within 2 seconds after completed thehot-rolling at cooling speeds of from 100 to 2,000° C./sec to cool thehot-rolled steel sheet to temperatures of from 600 to 750° C.; (5)applying the secondary cooling after the primary cooling at coolingspeeds of less than 50° C./sec, followed by applying coiling to thesecondary cooled hot-rolled steel sheet at temperatures of from 450 to650° C.

[0131] 3. The method for manufacturing steel sheet described in eitherof above-given 1 or 2, while further adding either one of the steps of:(1) applying annealing after pickling; and (2) applying cold-rollingafter pickling, followed by annealing.

[0132] 4. The method for manufacturing steel sheet described in eitherone of the above-given 1 through,3, in which the steel further contains0.01 to 0.3% as sum of one or more of Ti, Nb, V, Mo, Zr, and Cr.

[0133] According to the present invention, the composition and themanufacturing conditions are specified to attain the effect of theinvention. The detail of the reasons of specification is described inthe following.

[0134] 1. Composition

[0135] Carbon

[0136] Carbon is added to secure the strength of the steel sheet. If theC content is less than 0.05%, the strength of 340 MPa or more, which isa target of the present invention, cannot be attained. If the C contentexceeds 0.14%, the degradation of workability significantly degrades.Accordingly, the C content is specified to a range of from 0.05 to0.14%.

[0137] Silicon

[0138] Silicon is an element to strengthen the solid solution, thus S isadded to strengthen the steel sheet. If, however, the S content exceeds0.5%, the surface property degrades. Consequently, the S content isspecified to 0.5% or less.

[0139] Manganese

[0140] Manganese is added to 0.5% or more for improving the toughness ofthe steel sheet and to increase the strength by strengthening the solidsolution. If the Mn content exceeds 2.5%, the workability significantlydegrades. Therefore, the Mn content is specified to a range of from 0.5%to 2.5%.

[0141] Phosphorus

[0142] Phosphorus has a function to strengthen the solid solution tostrengthen the steel sheet. If, however, the P content exceeds 0.05%,the workability degrades owing to segregation. Consequently, the Pcontent is specified to 0.05% or less.

[0143] Sulfur

[0144] Sulfur forms sulfide, and the quantity of sulfide increases todegrade the workability if the S content exceeds 0.01%. Accordingly, theS content is specified to 0.01% or less.

[0145] Oxygen

[0146] Oxygen is specified to 0.005% or less to suppress crackgeneration on the surface of slab or under the surface layer of the slabduring continuous casting.

[0147] Calcium

[0148] Calcium converts alumina oxide, which is a deoxidized product inthe case of Al application for deoxidizing during steel meltmanufacturing stage, into a low melting point Al—Ca—O base oxide. Sincethe Al—Ca—O base oxide extends during hot-rolling to degrade theworkability (stretch flanging performance), the present invention treatsCa as an inevitable impurity. Consequently, Ca is not positively added,and the Ca content is specified to less than 0.005% which is a level ofnon-addition case.

[0149] The present invention deals with the above-given elements as thebasic composition components. Nevertheless, to further improve thecharacteristics, one or more of Ti, Nb, V, Mo, Zr, and Cr may further beadded.

[0150] Ti, Nb, V, Mo, Zr, Cr

[0151] According to the present invention, 0.01 to 0.3% as the sum ofone or more of Ti, Nb, V, Mo, Zr, and Cr can be added for improving thestrength.

[0152] According to the present invention, presence of elements otherthan those described above is allowable as far as they do not give badinfluence on the functions and effect of the present invention. Forexample, presence of 2% or less Cu and 0.04% or less Sn is allowable.

[0153] 2. Manufacturing Conditions

[0154] (1) Step of Forming Slab by Continuous Casting that ConductsTreatment to Reduce Segregation

[0155] To reduce the production cost and to manufacture slab at highyield, the present invention applies continuous casting.

[0156] During the casting stage, the treatment to reduce segregation isconducted to suppress the segregation of C, Mn, and the like during thecontinuous casting, to prevent the formation of a banded structure atcenter portion of the sheet thickness and the like, thus to attainexcellent workability (stretch flanging performance), combining with thecontrol of primary cooling speed after the finish-rolling (describedafter). Examples of the treatment to reduce segregation areelectromagnetic agitation, light draft casting, and increase in coolingspeed of ingot such as slab. These treatment methods can be appliedseparately or combined together.

[0157] (2) Step of Reheating the Slab before Hot-Rolling

[0158] For improving the uniformity of temperature in a slab, forhomogenizing the mechanical properties in the coil width direction, andfor further improving the workability, it is preferable to reheat theslab after continuous casting without cooling thereof to roomtemperature and to start rough-rolling. The reheating temperature ispreferably not higher than 1,250° C.

[0159] (3) Step of Hot-Rolling Regulating the End Temperature of theFinish-Rolling to Ar₃ Transformation Point or Above

[0160] The end temperature of rolling at the finish-rolling mill isselected to Ar₃ transformation point or above to refine the ferriticgrains and the pearlite after the transformation, thus improving thestretch flanging performance and the shock resistance.

[0161] (4) Step of Starting Primary Cooling at Cooling Speeds of from100 to 2,000° C./sec Within 2 Seconds After the Hot-Rolling, and toConduct the Cooling to Temperatures of from 600 to 750° C.

[0162] The cooling (primary cooling) on runout table after thehot-rolling starts within 2 seconds, preferably within 1 second, afterthe finish-rolling for reducing the size of ferritic grains and ofpearlite after the transformation, thus improving the excellentworkability and shock resistance with high yield strength. FIG. 1 showsthe influence of the time to start cooling on the mechanical properties.In the case that the cooling started within 2 seconds after completingthe finish-rolling, excellent workability and high strength can beattained.

[0163] The cooling speed of the primary cooling is specified to refinethe ferritic grains and the pearlite after the transformation and toimprove the stretch flanging performance by the suppression of bandedstructure formation at center portion of the sheet thickness. The placeof banded structure corresponds to the C and Mn enriched portion duringthe solidification step. At ordinary cooling speeds of 100° C./sec orless, the temperature of transformation from austenite to ferrite islow, and the banded structure transforms slower than any other portion.As a result, lots of pearlite are formed in the banded structure todegrade the stretch flanging performance.

[0164] If the cooling speed is 100° C./sec or more, the ferritetransformation becomes easy even in the C and Mn enriched portion, whichgives homogeneous elements distribution to suppress the banded structureformation. Higher cooling speed is more preferable. In view ofindustrial applicability, however, the upper limit of the cooling speedis 2,000° C./sec. For the case of Comparative Method that applies thecooling speed outside of the range of present invention, the bandedstructure is observed, and the grain size is larger than that of themicrostructure of the method of the present invention.

[0165] From the standpoint of refining the ferritic grains and thepearlite, the cooling speed is preferably 200° C./sec or more, and morepreferably 400° C./sec or more for further improving the workability.

[0166] If the end temperature of the primary cooling is higher than 750°C., the ferritic grain refinement becomes difficult. And if it is lessthan 600° C., the secondary phase becomes a hard low temperaturetransformation phase. Therefore, the end temperature of the primarycooling is specified to a range of from 600° C. or more and less than750° C.

[0167] (5) Step of Applying Secondary Cooling after the Primary Coolingat Cooling Speeds of less than 50° C./sec, then to Apply Coiling atTemperatures of from 450 to 650° C.

[0168] Succeeding to the primary cooling, the secondary cooling isapplied. The secondary cooling may be given immediately after the stopof the primary cooling or by given after a certain period of time tostand for cooling. That is, the time to start the secondary cooling isnot specifically specified. The cooling speed of the secondary coolingis specified to 50° C./sec or less to let the austenite structureadequately transform into pearlite structure to give excellentworkability.

[0169] The coiling temperature is regulated to a range of from 450 to650° C. because the coiling temperatures above 650° C. induces formationof pearlite which is harmful to ductility and because the temperaturesbelow 450° C. induces formation of low temperature transformed phase todegrade the workability. When further homogenized mechanical propertiesare wanted, the temperature difference in a coil is preferably to beregulated within 50° C. by applying, for example, a cooling unit havingexcellent cooling controllability.

[0170] On applying the present invention, application of pickling andannealing, or pickling, cold-rolling, and annealing after manufacturedthe hot-rolled steel sheet does not degrade the effect of the presentinvention. Furthermore, the effect of the present invention is notdegraded even when a hot dip zinc-coated material is used as substrateof hot-rolling and cold-rolling.

[0171] In addition, on applying the present invention, application of aninduction heating unit after the rough-rolling, before thefinish-rolling, or between the stands of finish-rolling to heat the edgeportions in width direction of coil gives further homogenized mechanicalproperties. Furthermore, the effect of the present invention is notharmed even under continuous hot-rolling in which the sheet bar iswelded after the rough-rolling followed by continuous finish-rolling.

[0172] Example

[0173] After the melt preparation of steels having the chemicalcompositions shown in Table 6 according to the present invention,hot-rolled steel sheets having a thickness of 2.0 mm were manufacturedusing the manufacturing method given in Table 7. For the materials Nos.1 and 2 and Nos. 5 through 9, the mechanical properties in as-hot-rolledstate were determined. For the material No. 3, the mechanical propertieswere determined after hot-rolled, pickled, cold-rolled, and hot dipgalvanized. For the material No. 4, the mechanical properties weredetermined after hot-rolled, pickled, and hot dip galvanized. As theevaluation of stretch flanging performance, the bore expanding rate (λ)was determined. Table 7 also gives the evaluation result.

[0174] The materials Nos. 1 through 4 as the Examples of the presentinvention, satisfying the chemical compositions and manufacturingconditions of the present invention were compared with the materialsNos. 5 through 9 as the Comparative Examples failing to satisfy eitherone of the manufacturing conditions of the present invention. Thematerials of Examples of the present invention definitely superior inworkability (balance of strength and bore expanding rate), high yieldstrength, and superior shock resistance. FIG. 2 shows the tensilestrength and the bore expanding rate of both the Examples and theComparative Examples. It is clearly shown that the present inventionprovides excellent characteristics. TABLE 6 Chemical composition (wt. %)C Si Mn S P O N Ca Remark 0.059 0.01 1.23 0.007 0.013 0.0023 0.0037 —Example

[0175] TABLE 7 End Slab End Time to start Primary temperature ofTreatment temperature the primary cooling the primary Heat to reduce ofrolling cooling speed cooling Classification Material No. historysegregation (° C.) (sec) (° C./sec) (° C.) Example 1 heating to Applied(Ar3)˜ 1.5 210 650 1250° C. (Ar3 + 30) 2 heating to Applied (Ar3)˜ 0.3200 680 1250° C. (Ar3 + 20) 3 heating to Applied (Ar3)˜ 0.3 200 6801250° C. (Ar3 + 20) 4 heating to Applied (Ar3)˜ 0.3 200 680 1250° C.(Ar3 + 20) Comparative 5 heating to Not (Ar3 + 10)˜ 0.3 205 670 example1250° C. applied* (Ar3 + 30) 6 heating to Applied (Ar3 + 10)˜ 0.6  30*650 1250° C. (Ar3 + 20) 7 heating to Applied (Ar3 + 10)˜ 0.6 205  550*1250° C. (Ar3 + 30) 8 heating to Applied (Ar3 + 5)˜ 0.6 195 680 1250° C.(Ar3 + 30) 9 heating to Applied (Ar3 + 10)˜ 0.6 195 690 1250° C. (Ar3 +20) Secondary cooling Coiling Mechanical properties speed temperature YSTS EL λ Classification Material No. (° C./sec) (° C.) (° C.) (MPa) (%)(%) Remark Example 1 40 600 382 451 35.2 115 Hot-rolled material 2 35605 397 470 32.5 110 Hot-rolled material 3 35 605 379 446 36.2 120Cold-rolled and galvanized material 4 35 605 387 456 35 116 Hot-rolledgalvanized material Comparative 5 40 600 395 471 31.5 91 Hot-rolledexample material 6 35 610 353 427 32 108 Hot-rolled material 7 20 605402 485 26 88 Hot-rolled material 8 35  660* 346 431 31.5 107 Hot-rolledmaterial 9 40  430* 397 480 26.5 91 Hot-rolled material

[0176] Preferred Embodiment 3

[0177] The inventors of the present invention conducted detail study onthe compositions, the rolling conditions, and the cooling conditionsafter the rolling, and found that the stability of strengthcharacteristics are particularly influenced by the cooling conditionsafter the rolling. Thus the inventors derived the present invention.That is, the present invention provides:

[0178] 1. A method for manufacturing high tension steel sheet comprisingthe steps of: hot-rolling a steel consisting essentially of 0.03 to0.12% C, 1% or less Si, 0.5 to 2% Mn, 0.02% or less P, 0.01% or less S,further at least one element selected from the group consisting of 0.005to 0.1% Nb, 0.005 to 0.1% V, and 0.005 to 0.1% Ti, by weight, attemperatures of 1,070° C. or less to accumulated reductions in thicknessof 30% or more; and cooling the hot-rolled steel sheet within 6 secondsafter completing the rolling at average cooling speeds of not less than80° C./sec to temperatures of above 500° C. and not more than 700° C.

[0179] 2. A method for manufacturing high tension steel sheet comprisingthe steps of: hot-rolling a steel consisting essentially of 0.03 to0.12% C, 1% or less Si, 0.5 to 2% Mn, 0.02% or less P, 0.01% or less S,and 0.05 to 0.5% Mo, by weight, at temperatures of 1,070° C. or less toaccumulated reductions in thickness of 30% or more; and cooling thehot-rolled steel sheet within 6 seconds after completing the rolling ataverage cooling speeds of not less than 80° C./sec to temperatures ofabove 500° C. and not more than 700° C.

[0180] 3. The method for manufacturing high tension steel sheet ofdescribed in above-given 1, wherein the steel further contains 0.05 to0.5% Mo.

[0181] The reasons to specify the compositions and the manufacturingconditions according to the present invention are described below.

[0182] 1. Composition

[0183] Carbon

[0184] Carbon is added to secure the strength of the steel sheet. If theC content is less than 0.03%, the effect cannot be attained. If the Ccontent exceeds 0.12%, the formation of low temperature transformationphase occurs to excessively increase the strength. Accordingly, the Ccontent is specified to a range of from 0.03 to 0.12%.

[0185] Silicon

[0186] Silicon is added to enhance the ferrite precipitation and toprevent excessive increase in YS. If, however, the S content exceeds 1%,the weldability degrades. Consequently, the S content is specified to 1%or less.

[0187] Manganese

[0188] Manganese is added for strengthening the solid solution, forimproving hardenability, and for improving the strength. If the Mncontent is less than 0.5%, the effect cannot be attained. If the Mncontent exceeds 2%, the workability degrades and the toughness degradesowing to the increase in the low temperature transformation phase.Therefore, the Mn content is specified to a range of from 0.5% to 2%.

[0189] Phosphorus and Sulfur

[0190] Since these elements degrade the toughness of steel, the Pcontent is specified to 0.02% or less and the S content is specified to0.01% or less.

[0191] According to the present invention, one or more of Nb, V, Ti, andMo are added to improve the strength.

[0192] Nb, V, Ti

[0193] The elements Nb, V, and Ti are the precipitation hardeningelements, and they establish fine microstructure of hot-rolled steelsheet to increase the strength. To give the effect, each of theseelement is added to 0.005% or more. Excessive amount of these elementssaturates the effect and degrades the weldability, and further degradesthe toughness owing to the increase in low temperature transformationphase. Therefore, the upper limit of the addition of each of theseelement is specified to 0.1%.

[0194] Molybdenum

[0195] Molybdenum improves the hardenability, strengthens the structure,and increases the strength. To attain the effect, Mo is added to 0.05%or more. However, excessive addition of Mo degrades the weldability andthe toughness owing to the increase in low temperature transformationphase. Consequently, the Mo content is specified to 0.5% or less.

[0196] According to the present invention, presence of elements otherthan those described above is allowable as far as they do not give badinfluence on the functions and effect of the present invention. Forexample, presence of 0.1% or less Al, Cu, Ni, B, Ca or the like and0.05% or less B and Ca is allowable.

[0197] 2. Rolling Condition

[0198] To establish uniform fine microstructure of hot-rolled steel bythe rolling in recrystallization temperature region, the rolling isconducted at temperatures of 1,070° C. or below with cumulativereduction in thickness of 30% or more.

[0199] 3. Cooling Condition

[0200] Time to Start Cooling

[0201] To refine the grains and to stabilize the strength and thetoughness, the cooling is started within 6 seconds after completed therolling. For improving the strength and the toughness by the grainrefinement effect, preferably the time to start cooling is within 3seconds.

[0202] Average Cooling Speed

[0203] The cooling speed is the most important variable in the presentinvention. Rapid cooling is adopted to prevent formation of coarsegrains and to assure homogeneous fine grains, with the average coolingspeeds of 80° C./sec or more, preferably 100° C./sec or more.

[0204] Temperature to Stop Cooling

[0205] When the temperature to stop cooling is low, the low temperaturetransformed phase increases and the YS significantly increases toexcessively increase the YR and to degrade the toughness. Therefore thetemperature to stop cooling is specified to 500° C. or more. On theother hand, if the temperature to stop cooling exceeds 700° C., thestability of strength cannot be obtained. Consequently, the temperatureto stop cooling is specified to a range of from higher than 500° C. tonot higher than 700° C.

[0206] According to the present invention, the steps after the stop ofthe rapid cooling are not specifically specified. In the case thatwinding is applied to form a coil, the process follows common practiceto apply slow cooling by air cooling or by runout table cooling followedby coiling. In that case, the slow cooling gives preferable effect ofreducing the formation of low temperature transformation phase and ofsuppressing excessive increase in YS value, thus, particularly the slowcooling at 40° C./sec or less is preferred.

[0207] On applying the present invention, application of an inductionheating unit at inlet of the continuous hot finish-rolling mill, orbetween the stands of the continuous hot finish-rolling mill to heat thesheet bar, and further application of an induction heating unit betweenthe stands of the continuous hot finish-rolling mill or the precedingstep to the finish-rolling mill to heat the edge portions in widthdirection of the sheet bar assure the homogenization of mechanicalproperties, thus the heating does not induce problem.

[0208] When the present invention is applied to a continuous hot-rollingprocess using a coil box, the heating of sheet bar may be given beforeor after the coil box or before or after the roughing mill, or after thecoil box, or before or after the welder, without raising problem.

[0209] Example

[0210] With the steels satisfying the chemical compositions,given inTable 8 according to the present invention, the influence of thevariations in manufacturing conditions on the strength characteristicswas investigated. The manufacturing conditions were varied in terms ofthe temperature to stop the primary cooling, which are given in Table 9.The primary cooling in the table expresses the rapid cooling after therolling, and the secondary cooling therein expresses the slow coolingafter the stop of the primary cooling and before the coiling.

[0211] Regarding the specimens Nos. 1 through 6, No. 1 and No. 6 are theComparative Examples giving the temperatures to stop the primary coolingabove 500° C. and not more than 700° C., which are outside of the rangeof the present invention. The manufacturing conditions of the specimensNos. 2 through 5 are within the range of the present invention, varyingthe temperature to stop the primary cooling, showing the Examples of thepresent invention. All the specimens had 7 mm in sheet thickness. Theresult of mechanical properties determination is shown in Table 10.FIGS. 3 through 7 show the result of mechanical property test given inTable 10. The specimens given in FIGS. 3 through 7 corresponded to 150°C./sec of the primary cooling speed and to 3° C./sec of the secondarycooling speed. In the figures, the rapid cooling expresses the primarycooling.

[0212] As clearly seen in the tables and figures, according to theconditions within the range of the present invention, the variations instrength characteristics of the obtained steel sheets are less toprovide stable characteristics even under varied manufacturingconditions. TABLE 8 C Si Mn P S Nb V Ti 0.08 0.25 1.57 0.006 0.00090.034 0.072 0.039

[0213] TABLE 8 Temperature Time to Primary to stop the Secondary HeatingRolling: Finishing start cooling primary cooling Coiling temperature1070° C. or temperature cooling speed cooling speed temperature Specimen(° C.) below (° C.) (sec) (° C./sec) (° C.) (° C./sec) (° C.) Remark 11230 47 → 7 820 — —  820* 3 570 C mmt 2 1230 47 → 7 820 0.6 150 660 3570 E mmt 3 1230 47 → 7 820 0.6 150 640 3 570 E mmt 4 1230 47 → 7 8200.6 150 570 — 570 E mmt 5 1230 47 → 7 820 0.6 150 520 — 520 E mmt 6 123047 → 7 820 0.6 150  450* — 450 C mmt

[0214] TABLE 10 YS TS El TS · El YR vTrs Specimen (MPa) (MPa) (%) (MPa ·%) (%) (° C.) 1 612 652 30 19560 93.9 −105 2 695 800 26.5 21200 86.9−115 3 688 795 26 20670 86.5 −105 4 685 797 25.8 20004 85.9 −110 5 699806 24.2 19650 86 −100 6 808 836 18.5 15466 96.7 −85

What is claimed is:
 1. A method for manufacturing steel sheet comprisingthe steps of: rough-rolling a continuously cast slab consistingessentially of 0.8% or less C, by weight to form a sheet bar;finish-rolling the sheet bar at finishing temperatures of (Ar₃transformation point −20° C.) or more to form a steel strip; rapidcooling the steel strip after completed the finish-rolling at coolingspeeds of higher than 120° C./sec down to temperatures of from 500 to800° C.; and coiling the steel strip after completed the rapid coolingat coiling temperatures of from 400 to 750° C.
 2. The method of claim 1,wherein the continuously cast slab contains 0.8% or less C, 2.5% or lessSi, and 3.0% or less Mn, by weight.
 3. The method of claim 1, whereinthe continuously cast slab contains 0.8% or less C, 2.5% or less Si,3.0% or less Mn, and 0.01 to 0.2% of at least one element selected fromthe group consisting of Ti, Nb, V, Mo, Zr, and Cr, by weight.
 4. Themethod of claim 1, wherein the continuously cast slab contains 0.8% orless C, 2.5% or less Si, 3.0% or less Mn, and 0.005% or less of at leastone element selected from the group consisting of Ca and B, by weight.5. The method of claim 1, wherein the continuously cast slab contains0.8% or less C, 2.5% or less Si, 3.0% or less Mn, 0.01 to 0.2% of atleast one element selected from the group consisting of Ti, Nb, V, Mo,Zr, and Cr, and 0.005% or less at least one element selected from thegroup consisting of Ca and B, by weight.
 6. The method of claim 1,wherein the rough-rolling of the continuously cast slab is carried outby direct hot-rolling.
 7. The method of claim 1, wherein therough-rolling of the continuously cast slab is carried out by reheatingthe slab to temperatures of 1,200° C. or below before cooling thereof toroom temperature.
 8. The method of claim 1, further comprising the stepof heating the sheet bar by an induction heating unit immediately beforethe finish-rolling or during the finish-rolling.
 9. The method of claim1, wherein the rapid cooling of the steel strip is started within a timeranging from more than 0.1 second and less than 1 second after completedthe finish-rolling.
 10. The method of claim 1, further comprising thesteps of: cold-rolling the coiled steel strip; and annealing thecold-rolled steel strip.
 11. The method of claim 1, wherein the rapidcooling step is carried out so as the temperature difference between themaximum value and the minimum value in width direction and inlongitudinal direction of the steel strip after the rapid cooling tobecome 60° C. or less.
 12. The method of claim 1, wherein the rapidcooling step is carried out by cooling the steel strip at heat transfercoefficients of 2,000 kcal/m²h ° C. or more.
 13. A steel sheet preparedby the method for manufacturing steel sheet of claim 1 and havingvariations of tensile strength in width direction and in longitudinaldirection thereof within ±8% of average value of the tensile strength ina coil.
 14. A method for manufacturing steel sheet comprising the stepsof: rough-rolling a continuously cast slab consisting essentially ofmore than 0.8% and 1% or less C, by weight to form a sheet bar;finish-rolling thus obtained sheet bar at finishing temperatures of (Acmtransformation point −20° C.) or more to form a steel strip; rapidcooling the steel strip after completed the finish-rolling at coolingspeeds of higher than 120° C./sec down to temperatures of from 500 to800° C.; and coiling the steel strip after completed the rapid coolingat coiling temperatures of from 400 to 750° C.
 15. The method of claim14, wherein the rough-rolling of the continuously cast slab is carriedout by direct hot-rolling.
 16. The method of claim 14, wherein therough-rolling of the continuously cast slab is carried out by reheatingthe slab to temperatures of 1,200° C. or less before cooling thereof toroom temperature.
 17. The method of claim 14, further comprising thestep of heating the sheet bar by an induction heating unit immediatelybefore the finish-rolling or during the finish-rolling.
 18. The methodof claim 14, wherein the rapid cooling of the steel strip is startedwithin a time ranging from more than 0.1 second and less than 1 secondafter completed the finish-rolling.
 19. The method of claim 14, furthercomprising the steps of: cold-rolling the coiled steel strip; andannealing the cold-rolled steel strip.
 20. The method of claim 14,wherein the rapid cooling step is carried out so as the temperaturedifference between the maximum value and the minimum value in widthdirection and in longitudinal direction of the steel strip after therapid cooling to become 60° C. or less.
 21. The method of claim 14,wherein the rapid cooling step is carried out by cooling the steel stripat heat transfer coefficients of 2,000 kcal/m²h° C. or more.
 22. A steelsheet prepared by the method for manufacturing steel sheet of claim 14and having variations of tensile strength in width direction and inlongitudinal direction thereof within ±8% of average value of thetensile strength in a coil.
 23. A method for manufacturing steel sheetcomprising the steps of: forming a slab consisting essentially of 0.05to 0.14% C, 0.5% or less Si, 0.5 to 2.5% Mn, 0.05% or less P, 0.01% orless S, 0.005% or less O, and less than 0.0005% Ca, by weight, bycontinuous casting conducting treatment to reduce segregation;hot-rolling the slab at finishing temperatures of finish-rolling of Ar₃transformation point or more to form a hot-rolled steel sheet; startingprimary cooling within 2 seconds after completed the hot-rolling atcooling speeds of from 100 to 2,000° C./sec to cool the hot-rolled steelsheet to a temperature range of from 600 to 750° C.; applying secondarycooling, after cooling to the temperature range, at cooling speeds ofless than 50° C./sec; and coiling the secondary-cooled hot-rolled steelsheet at temperatures of from 450 to 650° C.
 24. The method of claim 23,further comprising the step of reheating the slab before the hot-rollingthereof.
 25. The method of claim 23, further comprising the steps of:pickling the coiled hot-rolled steel sheet; and annealing the pickledhot-rolled steel sheet.
 26. The method of claim 23, further comprisingthe steps of: pickling the coiled hot-rolled steel sheet; cold-rollingthe pickled hot-rolled steel sheet; and annealing the cold-rolled steelsheet.
 27. The method of claim 23, wherein the slab further contains0.01 to 0.3% at least one element selected from the group consisting ofTi, Nb, V, Mo, Zr, and Cr, by weight.
 28. A method for manufacturingsteel sheet comprising the steps of: hot-rolling a steel consistingessentially of 0.03 to 0.12% C, 1% or less Si, 0.5 to 2% Mn, 0.02% orless P, 0.01% or less S, further at least one element selected from thegroup consisting of 0.005 to 0.1% Nb, 0.005 to 0.1% V, and 0.005 to 0.1%Ti, by weight, at temperatures of 1,070° C. or below to accumulatedreductions in thickness of 30% or more; and cooling the hot-rolled steelsheet within 6 seconds after completed the rolling at average coolingspeeds of not less than 80° C./sec to temperatures of from above 500° C.to not more than 700° C.
 29. The method of claim 28, wherein the steelfurther contains 0.05 to 0.5% Mo.
 30. A method for manufacturing steelsheet comprising the steps of: hot-rolling a steel consistingessentially of 0.03 to 0.12% C, 1% or less Si, 0.5 to 2% Mn, 0.02% orless P, 0.01% or less S, and 0.05 to 0.5% Mo, by weight, at temperaturesof 1,070° C. or below to accumulated reductions in thickness of 30% ormore; and cooling the hot-rolled steel sheet within 6 seconds aftercompleted the rolling at average cooling speeds of not less than 80°C./sec to temperatures of from above 500° C. to not more than 700° C.